Bang bang, you're boolean!

If you see code like this:

function isLoggedIn() {  return user !== undefined;}

You can refactor it to:

function isLoggedIn() {  return !!user;}

A double negation !! converts an object to boolean in accordance with type coercion rules. If the object is one of eight Falsy values defined in JavaScript specification (undefined, null, false, NaN, 0, -0, 0n, ""), the result will be false. Otherwise, it will be true.

Remember it by:

Bang, bang, you're boolean!

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